1. Field of Invention
This invention relates to a photothermographic silver halide element and in particular to shelf-life and post-processing stabilization of photothermographic silver containing elements.
2. Background to the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic materials) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (1) a photosensitive material that generates atomic silver when irradiated, (2) a non-photosensitive, reducible silver source, (3) a reducing agent for the non-photosensitive, reducible silver source, and (4) a binder. The photosensitive material is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and a progenitor of the photosensitive photographic silver halide may be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions, such as by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049), and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. The preferred non-photosensitive reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image. This produces a black and white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
One conventional way of attempting to increase the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. Color images can be formed by incorporation of leuco dyes into the emulsion. Leuco dyes are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 3,351,286; 4,187,108; 4,426,441, 4,374,921; and 4,460,681.
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsions layer. Photothermographic elements having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well know in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
One common problem that exists with photothermographic systems is the instability of the image following processing. The photo active silver halide still present in the developed image may continue to catalyze print-out of metallic silver during room light handling. Thus, there exists a need for stabilization of the unreacted silver halide. The addition of separate post-processing image stabilizers or stabilizer precursors provides the desired post-processing stability. Most often these are sulfur-containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure 17029. U.S. Pat. No. 4,245,033 describes sulfur compounds of the mercapto-type that are development restrainers of photothermographic system. See also U.S. Pat. Nos. 4,837,141 and 4,451,561. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles, such as 3-amino-5-benzothio-1,2,4-triazole, used as post-processing stabilizers are described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure 16977 and 16979.
Thus, there has been a continued need for improved post-processing stabilizers that do not fog or desensitize the photographic materials, and stabilizer precursors that release the stabilizing moiety at the appropriate time and do not have any detrimental effects on the photosensitive material or user of the material.